7.10.5. Other Contaminants from Supersonic Transport Engines

At this stage, there is no reliable information relating specifically to the
design of supersonic propulsion combustion systems, but there is no reason to
believe that the degree of inefficiency should be any different from subsonic
types. Emissions arising from this form of inefficiency are discussed in Section
7.5. The fundamental nature of the proportion of CO2
and H2O emitted with conventional fuels and similar
proportions of sulfur in available fuels suggests that these emittants will
be produced in the same proportion to fuel usage as in the subsonic fleet, though
at supersonic cruise they will be deposited in the stratosphere.

7.10.6. Supersonic Transport Operations

Assessments of advanced supersonic aircraft concentrate on long-range over-water
routes but do not assume that such routes will be served exclusively by supersonic
services. It is now assumed that supersonic flight will occur only over water.
The combination of overland routes where sonic booms are unacceptable and fitting
into daily cycles will preserve a place for long-range subsonic services to
meet airport curfews and provide passengers with comfortable time zone changes.

7.10.7. Mitigation

Research programs to develop low NOx combustion
systems for cruise are in place in Europe, the United States, and Japan. Two
basic combustion concepts are being researched to produce ultra-low NOx
levels at supersonic cruise conditions. These technologies-the Lean Premixed
Prevaporized (LPP) and Rich Burn Quick Quench (RBQQ)-utilize both lean and rich
combustion concepts such as those shown in Figure 7-42
on the previous page. As a result of low combustor operating pressures in supersonic
transport applications, these concepts appear capable of achieving their full
ultra-low NOx reduction potential while maintaining
satisfactory durability and performance. In subsonic transports, the higher
pressures dictate compromises in both concepts to avoid incipient flashback
in the LPP and excessive soot production in the RBQQ.

The LPP concept has the likely potential of reaching the lowest levels of NOx.
The intent of premixing is to provide the combustion zone with a very lean,
uniform fuel/air mixture that is just above the flame extinction limit. This
approach results in a low flame temperature with enough residence time to complete
combustion and produce low NOx. Maintaining uniform
fuel/air mixtures throughout the combustor is critical because NOx
increases rapidly with any local fuel/air maldistribution. In practice, premixing
is achieved with large numbers of small-diameter premixers. Design challenges
with this concept include flashback or auto-ignition in the premixer, maintaining
combustion near the lean extinction limit over the entire engine cycle operating
span, potential fuel clogging of small-diameter fuel injectors, and complexity
of the design because of fuel staging requirements.

The RBQQ concept is a derivative of an axially staged combustor and presents
the more stable combustion configuration. The fuel/air mixture of the primary
combustion zone is fuel-rich, thus producing low flame temperatures and low
NOx. In a second stage, air is quickly introduced
to mix with the partially reacted fuel. Combustion is completed in a final stage
at lean conditions. Most of the NOx is produced
in the second stage and is a function of the uniformity and time it takes to
dilute the reacting mixture. Design challenges with this concept include indirect
cooling of the primary combustion zone-which may require high-temperature ceramic
materials currently under development-and an advanced second stage that produces
nearly instantaneous, uniform mixing of reacting gas and air. Furthermore, this
design may require engine power-related control of air and/or fuel staging for
practical implementation.

There are three active programs in which this research is being conducted:

U.S. NASA High Speed Civil Transport (HSCT), with a goal of supersonic cruise
NOx of EI(NOx)
= 5

Under laboratory and component test cell conditions, very low levels of NOx
below EI(NOx) = 5 have been achieved at simulated
engine operating conditions of pressure, temperature, and fuel flow with combustor
sectors. This result gives credence to the view that EI(NOx)
= 5 could well be achieved in engine tests scheduled after the year 2000. The
intrinsic outputs from combustion of conventional fuels are more difficult to
alleviate. With kerosene as a fuel, for every ton of fuel burned, 3.2 tons of
CO2 and 1.2 tons of water are produced.